Thermally developable light-sensitive layers containing photobleachable sensitizers

ABSTRACT

Negative-acting photothermographic imaging systems with improved color fidelity and light fastness are disclosed comprising a photobleachable sensitizer, a nitrate salt, an initiator, a leuco dye, a binder, and an optional organic acid. These systems may be used in a variety of applications comprising single or multiple layers in either single or multiple sheet constructions to provide color imaging elements.

CROSS-REFERENCE TO RELATED CASES

This is a continuation of application Ser. No. 07/539,565 filed Jun. 18,1990, now abandoned. Which is related to a commonly assigned applicationentitled "Light Sensitive Elements", U.S. Ser. No. 83,522 filed Aug. 7,1987, continued as U.S. Ser. No. 394,250, filed Aug. 11, 1989.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to visible light sensitive imagingsystems, and in particular to photothermographic imaging systemscomprising a nitrate salt, a leuco dye, an initiator, a sensitizing dye,a binder, and an optional organic acid.

2. Information Disclosure Statement

Many processes and compositions use leuco dyes to provide opticaldensities in the imaged article. For example, U.S. Pat. No. 4,017,313uses a combination of a photosensitive leuco dye, a photosensitizer forthe dye, an aromatic aldehyde and a secondary or tertiary amine. Otherphotosensitive systems using leuco dyes are included in U.S. Pat. Nos.3,390,997, 2,884,326, and 2,772,284. The mechanism of these last twopatents are disclosed in "Aromatic Aldehyde-Leuco Dye Photooxidation"(Hartzler, H. D. Pure Appl. Chem. 1977, 49, pp 353-356.)

Light-Sensitive Systems (Kosar, J.; John Wiley and Sons: New York, 1965:p 369), describes print-out photosensitive systems comprising a binder,leuco dye, organic halogen releasing compound and a photosensitizingdye. Because these are printout systems, there is no thermalamplification.

A great many photosensitive materials have been used in differentimaging processes utilizing various photoinitiated phenomena such asphotohardening of polymerizable materials (e.g., negative-actingprinting plates), photosolubilizing materials (e.g., positive-actingprinting plates), light initiated diazonium salt coupling reactions(e.g., diazonium microfilm), etc. A class of iodonium photoinitiatorsfor both cationic and epoxy polymerization (e.g., U.S. Pat. Nos.4,026,705 and 3,981,897) has also been proposed as equivalent to otherphotoinitiators in certain ethylenically unsaturated printing platecompositions (e.g., U.S. Pat. No. 3,741,769).

Photothermographic imaging systems are well known in the art. Bydefinition photothermographic systems are light-sensitive imagingsystems which are thermally developed. Photothermographic systemstypically require temperatures in the range of 80° to 200° C. A numberof imaging systems employ photosensitive compounds, leuco dyes orbleachable dyes, and nitrate salts to generate color images. Imagingsystems comprising a leuco dye or bleachable dye, nitrate ion, anddiazonium salts in a binder are disclosed in U.S. Pat. No. 4,370,401. Inthose cases wherein a leuco dye system is employed, aphotothermographic, negative-acting imaging system is provided; that is,the optical density in the final image is more dense in areas which arelight struck than in areas which are not light struck. Conversely, inthose cases wherein a bleachable dye system is employed, aphotothermographic, positive acting imaging system is provided. That is,the optical density in the final image is more dense in areas which arenot light struck than in areas which are light struck. The bleachabledye used in these cases does not serve in the role of a sensitizer.

Related imaging compositions comprising a diazonium salt and leuco dyein a binder are disclosed in U.S. Pat. No. 4,394,433. These unamplifiedcompositions are positive-acting photothermographic compositions, anddiffer fundamentally from the compositions of the present invention,which are amplified by the action of a nitrate salt.

Additional light-sensitive, thermally developable imaging systems areknown. U.S. Pat. No. 4,460,677 describes a thermally developable imagingsystem comprising a leuco dye, nitrate ion, and a spectrally sensitizedorganic compound having photolyzable halogen atoms. Similarly, U.S. Pat.No. 4,386,154 describes a thermally developable imaging systemcomprising a leuco dye, a nitrate salt, and a spectrally sensitizedcompound selected from (1) aromatic iodonium salts and (2) photolyzableorganic halogen compounds. Both of these compositions act as negativeimage forming systems in that the greatest image density is formed uponheat development in the light struck areas. The latent images are formedupon exposure to visible light and visible images are formed by heatdevelopment. The color fidelity and contrast of both of these systems isreduced by the presence of sensitizer stain, that is color due to thesensitizer which remains in the system, and to colored by-products fromreacted sensitizer in light exposed regions. This sensitizer stainaesthetically and functionally detracts from the image.

Light-sensitive, thermally developable imaging systems are alsodescribed in several Japanese patents.

Japanese Pat. No. 77,025,330 pertains to a two component positive actingimaging composition comprised of an oxazine or phenothiazine leuco dye(BLMB) mono- or disubstituted with a dialkylamino group, and anoxidizing agent such as nitrate ion. Said composition is not lightsensitive in the visible region of the spectrum, and does not employdiaryliodonium salts or organic compounds having photolyzable halogenatoms.

Japanese Pat. No. 77,004,180 describes the use of triplet sensitizersfor BLMB. Suitable sensitizers are aromatic carbonyl compounds andaromatic nitro compounds. That patent describes both negative andpositive systems, and is a counterpart to Japanese Pat. No. 77,025,330.Said composition is not light sensitive in the visible region of thespectrum, and does not employ diaryliodonium salts or organic compoundshaving photolyzable halogen atoms. Further, triplet sensitizers are bydefinition not photobleached during energy transfer.

Japanese Pat. No. 76,035,847 describes photosensitive heat-fixablerecording materials containing a free radical producing organic halogencompound, leuco dye and a base. This is a negative-acting system, andcontains no oxidizer.

Japanese Pat. No. 77,025,088 describes photosensitive compositionscontaining an acid-sensitive leuco dye (e.g., naphthospiropyran), and aphotochemical acid generating agent which is a mixture of an organichalide (e.g., CBr₄), with a furan containing compound.

Japanese Pat. No. 79,001,453 describes a photothermographic materialwhich contains an oxidizer, a compound which reacts with the oxidizer tochange or develop color, and a compound which deactivates the colordeveloper either in exposed or unexposed regions. Images can be eitherpositive or negative, and do not employ sensitizers or diaryliodoniumsalts or organic compounds having photolyzable halogen atoms, which arecomponents of the present invention, and which activate rather thandeactivate color development. The light sensitive materials used werecolorless or nearly colorless aryl quinones and ultraviolet lightsources were used for imaging. Additionally, the light-sensitivematerials used were not photobleachable. Furthermore, the compositionsof the present invention show considerable amplification in both exposedand unexposed regions due to the presence of an added UV sensitiveinitiator (i.e., diaryliodonium salt or a organic compound withphotolyzable halogen atoms), and therefore do not function bydeactivation in the sense of Japanese Pat. No. 79,001,453.

Decolorizable imaging systems comprising a binder, nitrate salt, acid,and dyes are disclosed in U.S. Pat. Nos. 4,336,323 and 4,373,020. Thesesystems are particularly useful as antihalation layers inphotothermographic systems where the development temperature acts tobleach the dye.

The use of photobleachable dyes including o-nitroarylidene dyes asantihalation or acutance dyes is known in the art: U.S. Pat. Nos.4,111,699; 4,271,263; 4,088,497; 4,033,948; 4,028,113; 3,988,156;3,988,154; 3,984,248; 3,615,432 (RE28,225). The use of photobleachabledyes in this manner is unrelated to their function in the presentinvention. Additionally, it was found that o-nitroarylidene dyes aredesensitizing to silver halide imaging systems.

Spectral sensitization of silver containing photothermographiccompositions has been disclosed in U.S. Pat. No. 4,461,828; however, nomention is made of photobleachable sensitizers.

U.S. Pat. No. 4,713,312 teaches photobleachable sensitizers useful inthe wavelength range of 390-500 nm for free radical polymerization toreduce background sensitizer stain in an imaging system based onphotosensitive microcapsules. In the case of a full color imagingsystem, for which sensitivity to the entire visible spectrum of 400-700nm is necessary, this patent does not provide for complete removal ofresidual sensitizer stain.

SUMMARY OF THE INVENTION

Photothermographic imaging layers of this invention comprise a nitratesalt, a leuco dye, an initiator, and a photobleachable sensitizer. Theinitiators useful for this invention include diaryliodonium salts andphotolyzable organic halogen compounds. The photobleachable sensitizersof this invention include, but are not limited to o-nitroarylidene dyessuch as 2,4-dinitrobenzylidene, 3,6-dinitropyridyl-2-idene, and5-nitropyrimidyl-4-idene containing dyes, and are capable of sensitizingan initiator over the entire visible spectrum of 400-700 nm.

This invention describes layers comprised of a nitrate salt, a leucodye, an initiator, and a photobleachable sensitizer. These layers arenormally carried by a binder such as a polymeric binder which may alsocontain an organic acid.

The imaging layers of this invention have reduced residual sensitizerstain in both exposed regions of the layer, and following apost-development blanket irradiation, in unexposed regions as well.Following a post-development blanket irradiation, the imaging layers ofthis invention may optionally be fixed by treatment with a basicmaterial to neutralize acid which is present in the layer.

This invention describes a photothermographic imaging layer whichprovides clean and stable images by overcoming the deficiencies of theprior art which are (1) undesirable residual sensitizer stain, and (2)relatively unstable images due to absorption of light and subsequentchanges in color.

This invention is achieved by providing novel photothermographic imaginglayers which comprise a leuco dye, nitrate salt, an initiator, and aphotobleachable sensitizer (preferably an o-nitroarylidene) compound.These layers are preferably carried in a binder such as a polymericbinder, and preferably also contain an organic acid. After exposing thesystem to light, the application of heat develops the image by oxidizingthe leuco dye more rapidly in the exposed region affording anegative-acting system. An image results because of the differentialrate of oxidation occurring in exposed and unexposed regions. Latentimages are formed upon exposure to visible light and stable images arethen formed by heat development and blanket light exposure. Therefore,no wet processing steps are needed.

DETAILED DESCRIPTION OF THE INVENTION

Thermally developable imaging systems described in U.S. Pat. Nos.4,460,677 and 4,386,154 comprise a leuco dye, nitrate ion, and either(1) a spectrally sensitized organic compound having photolyzable halogenatom; or (2) a spectrally sensitized diaryliodonium salt. Each of thesesystems has two disadvantages: (1) aesthetically undesirable residualsensitizer stain, and (2) relatively unstable images due to absorptionof light causing subsequent changes in color. Accordingly, there is aneed for the light-sensitive, thermally developable imaging layers ofthis invention which provide cleaner and more stable images.

There are a minimum of five components to the imaging systems of thepresent invention. The five required ingredients are (1) aphotobleachable spectral sensitizing dye (spectral sensitizer), (2) anitrate salt, (3) a leuco dye, (4) an initiator, and (5) polymeric resin(binder). An acidic material constitutes a preferred sixth ingredient.

PHOTOBLEACHABLE SENSITIZER

Any dye which is both a sensitizer for initiators of the presentinvention and which is also photobleachable is useful in the presentinvention. A particular class of dyes useful as photobleachablesensitizers of this invention is the class of o-nitro-substitutedarylidene dyes comprising an o-nitro-substituted aryl group joinedthrough a methine chain linkage, said methine chain having a minimum of2 methine carbon atoms, to an electron donor; said donor comprisingeither (1) a basic heterocyclic nucleus containing an electron-donatingatom, typically a nitrogen, oxygen, or sulfur electron-donating atom, or(2) an alkylamino group, or (3) a dialkylamino group. The number ofatoms joining the basic heterocyclic nucleus and the aryl group can bean even or odd number. As employed herein the term "arylidene" refers toa group formed by an aryl group and a methine linkage (e.g.,benzylidene, cinnamylidene, etc.)

The term photobleachable means that upon exposure to actinic radiationbetween about 350 nm and about 1100 nm the dye is converted to acolorless or nearly colorless form (i.e., the molar absorptivity isreduced by at least a factor of 5). Photobleachable sensitizers usefulin the present invention bleach at least 10%, and preferably bleach atleast 25% and more preferably at least 50% when exposed to the followingconditions:

a film of polyethylene terephthalate (4 mil thickness) is coated withthe sensitizer in question so as to create a colored film with anabsorbance of from 0.1 to 0.6, whereupon said colored film is thenplaced onto the Fresnel lens of a 3M brand Model 213 Overhead Projectorand exposed to light therefrom for 5 minutes.

The photobleachable sensitizers are said to bleach at a given percentagewhen the layer containing the sensitizer decreases absorbance(absorption intensity) by a given percentage at the longest wavelengthabsorption band maximum. This absorbance may be measured either bypercentage reduction in optical density provided by the sensitizer or bymeasurement of the percentage of radiation actually absorbed.

The overhead projector uses a single General Electric 82V ENX 360Wprojection bulb having a color temperature of 33300° K. The lightintensity on the image stage is 0.46 W/cm² ±0.05 W/cm².

In a preferred embodiment, the o-nitro-substituted aryl group is joinedthrough an acyclic methine chain containing an even or odd number ofmethine groups to a 5- or 6-membered basic, cyanine dye-typeheterocyclic nucleus. The heterocyclic nucleus can have additionalcarbocyclic and heterocyclic rings fused thereto. Theo-nitro-substituted aryl group can contain a phenyl or heterocyclicnucleus, or can contain a nucleus formed by fused aromatic orheteroaromatic rings, such as naphthyl and the like. U.S. Pat. Nos.3,984,248, 3,988,154, 3,988,156, and 4,271,263 disclose certain membersof the o-nitroarylidene dyes as acutance agents in thermally-developablephotosensitive layers. U.S. Pat. No. 4,095,981 discloses certain membersof the o-nitroarylidene dyes as energy sensitive dyes in silver basedphotographic or photothermographic materials.

In a specific preferred embodiment of this invention, theo-nitro-substituted dyes have the general Formula 1: ##STR1## wherein krepresents 0 or 1;

m represents 0 or 1;

each L represents a methine group, including substituted methine groups(e.g., --CH═, --C(CH₃)═, etc.);

A represents an electron donating moiety, such as oxygen (--O--), sulfur(--S--), or ##STR2## R₁ represents (1) an alkyl group having from 1 to18 carbon atoms and preferably a lower alkyl group having from 1 to 4carbon atoms (e.g., methyl, ethyl, propyl, isopropyl, butyl, sec-butyl,isobutyl, tert-butyl); a sulfoalkyl group, preferably sulfo lower alkylcontaining from 1 to 4 carbon atoms in the alkyl moiety (e.g.,β-sulfoethyl, γ-sulfopropyl, γ-sulfobutyl, etc.); a carboxyalkyl group,preferably a carboxy lower alkyl containing from 1 to 4 carbon atoms inthe alkyl moiety (e.g., β-carboxyethyl, γ-carboxypropyl, δ-carboxybutyl,etc.); a sulfatoalkyl group, preferably a sulfato lower alkyl containing1 to 4 carbon atoms in the alkyl moiety (e.g., β-sulfatoethyl,γ-sulfatopropyl, δ-sulfatobutyl, etc.); an alkoxyalkyl group, preferablya lower alkoxy lower alkyl containing from 1 to 4 carbon atoms in boththe alkoxy and alkyl moieties (e.g., β-methoxyethyl, γ-methoxypropyl,δ-propoxybutyl, etc.); an acyloxyalkyl group preferably an acyloxy loweralkyl containing from 1 to 4 carbon atoms in the alkyl moiety (e.g.,acetyloxyethyl, propanoyloxyethyl, butanoyloxybutyl, benzoyloxyethyl,toluyloxypropyl, etc.); an alkoxycarbonylalkyl group, preferably a loweralkoxy carbonyl lower alkyl containing from 1 to 4 carbon atoms in boththe alkoxy and alkyl moieties (e.g., β-methoxycarbonylethyl,δ-ethoxycarbonylbutyl, β-butoxycarbonylethyl, etc.); adialkylaminoalkylene group, preferably a di-lower alkylamino loweralkylene containing from 1 to 4 carbon atoms in the alkylene and thealkyl moieties (e.g., dimethylaminoethylene, diethylaminopropylene,diethylaminobutylene, etc.); a cycloaminoalkylene group, preferablycycloamino lower alkyl containing 4 to 6 atoms in the cycloamino moietyand 1 to 4 atoms in the alkyl moiety (e.g., pyrrolidinylethylene,morpholinopropylene, piperidinebutylene, pyrrolidinylmethylene, etc.);(2) an alkenyl group (including a substituted alkenyl group), preferablya lower alkenyl containing 2 to 4 carbon atoms (e.g., ethyl, allyl,1-propenyl, 1-butenyl, 2-butenyl, etc.); or (3) an aryl group (includinga substituted aryl), such as phenyl, naphthyl, tolyl, xylyl, halophenyl(e.g., p-chlorophenyl, p-bromophenyl, etc.), alkoxyphenyl (such asmethoxyphenyl, 2,4-dichlorophenyl, etc.), and an alkyl group, preferablyan aryl lower alkyl containing from 1 to 4 carbon atoms in the alkylmoiety (e.g., benzyl, β-phenethyl, ω-phenbutyl, etc.); or (4) hydrogen;and

Y represents the atoms necessary to complete an aryl (preferably phenylor naphthyl) ring which is o-nitro-substituted and preferably is alsop-substituted with a nitro or other electron withdrawing group and whicharyl ring can have other substituents attached to it and othercarbocyclic rings fused to it (e.g., 2-nitrophenyl, 2,4-dinitrophenyl,2,6-dinitrophenyl, 2,4,6-trinitrophenyl, 2--nitronaphthyl,2,4-dinitronaphthyl, 2-nitro-4-cyanophenyl,2-nitro-4-ethoxycarbonylphenyl, 2-nitro-4-trifluoromethylphenyl, and thelike); and

Z represents the nonmetallic atoms necessary to complete a heterocyclicnucleus of the type used in cyanine dyes containing 5 or 6 atoms in theheterocyclic ring containing the electron-donating atom of the formulawhich ring can contain a second heteroatom such as oxygen, nitrogen,selenium, or sulfur. The heterocyclic nucleus preferably is selectedfrom the group consisting of thiazole nucleus including substituted andunsubstituted benzothiazole and naphthothiazole nuclei and like (e.g.,thiazole, 4-methylthiazole, 4-phenylthiazole, 4,5-diphenylthiazole,4-(2-thienyl)thiazole, benzothiazole, 4-chlorobenzothiazole,4-methylbenzothiazole, 4-methoxybenzothiazole, 4-ethoxybenzothiazole,4-phenylbenzothiazole, 5-chlorobenzothiazole, 5-bromobenzothiazole,5-methylbenzophenylbenzothiazole, 5-methoxybenzothiazole,5-ethoxybenzothiazole, 6-chlorobenzothiazole, 6-ethoxybenzothiazole,5-methoxynaphtha[2,3-d]thiazole, 5-nitrobenzothiazole,6-nitrobenzothiazole, 5-chloro-6-nitrobenzothiazole, etc.); an oxazolenucleus including substituted and unsubstituted benzoxazole andnaphthoxazole nuclei and the like (e.g., oxazole, 4-phenyloxazole,benzoxazole, 5-chlorobenzoxazole, 5-methylbenzoxazole,5-bromobenzoxazole, 5-methoxybenzoxazole, 5-ethoxybenzoxazole,5-phenylbenzoxazole, 5-nitrobenzoxazole, 6-nitrobenzoxazole,5-chloro-6-nitrobenzoxazole, etc.); a selenazole nucleus includingsubstituted or unsubstituted benzoselenazole and naphtoselenazole nucleiand the like (e.g., selenazole, 4-methylselenazole, 4-nitroselenazole,4-phenylselenazole, benzoselenazole, 5-chlorobenzoselenazole,6-chlorobenzoselenazole, naphtho[2,1-l]selenazole,5-nitrobenzoselenazole, 6-nitrobenzoselenazole,5-chloro-6-nitrobenzoselenazole, nitro-group substitutednaphthoselenazoles, etc.); a thiazoline nucleus (e.g., thiazoline,4-methylthiazoline, 4-nitrothiazoline, etc.); a 2-pyridine nucleus,(e.g., 2-pyridine, 5-methyl-2-pyridine, etc.); a 4-pyridine nucleus(e.g., 4-pyridine, 3-methyl-4-pyridine, nitro-group substitutedpyridines, etc.); a 3,3-dialkylindolenine nucleus (e.g.,3,3-dimethylindolenine, 3,3-diethyl-5- or 6-cyanoindolenine,3,3-diethyl-5- or 6-nitroindolenine, 3,3-dimethyl-5- or6-nitroindolenine, etc.); an imidazole nucleus (e.g., imidazole;1-alkylimidazole; benzimidazole; 1,3-dialkyl, 1,3-diaryl, or1-alkyl-3-arylimidazoles and benzimidazoles (e.g.,5-chloro-1,3-dialkylbenzimidazoles, 5-chloro-1,3-diarylbenzimidazoles,5-methoxy-1,3-dialkylbenzimidazoles, 5-methoxy-1,3-diarylbenzimidazoles,5-cyano-1,3-dialkylbenzimidazoles, 5-cyano-1,3-diarylbenzimidazoles,1,3-dialkylnaphth[1,2-d]imidazole, 1,3-diarylnaphth[1,2-d]imidazole),etc.); a quinoline nucleus (e.g., quinoline, 6-methylquinoline,6-methoxyquinoline, 6-ethoxyquinoline, 6-ethoxyquinoline,6-chloroquinoline, 4-methoxyquinoline, 4-methylquinoline,8-methoxyquinoline, 2-methylquinoline, 4-chloroquinoline,6-nitroquinoline, etc.); an imidazo[4,5-b]quinoxaline nucleus (e.g.,imidazo[4,5-b]quinoxaline, 1,3-dialkylimidazo[4,5-b]quinoxaline such a1,3-diethylimidazo[4,5-b]quinoxaline,6-chloro-1,3-diethylimidazo[4,5-b]quinoxaline, etc.;1,3-dialkenylimidazo[4,5-b]quinoxaline such as1,3-diallylimidazo[4,5-b]quinoxaline,6-chloro-1,3-diallylimidazo[4,5-b]quinoxaline, etc.;1,3-diarylimidazo[4,5-b]quinoxaline such as1,3-diphenylimidazo[4,5-b]quinoxaline,6-chloro-1,3-diphenylimidazo[4,5-b]quinoxaline, etc.); a3H-pyrrolo[2,3-b]pyridine nucleus, (e.g.,3,3-dialkyl-3H-pyrrolo[2,3-b]pyridine such as3,3-dimethyl-3H-pyrrolo[2,3-b]pyridine,3,3-diethyl-3H-pyrrolo[2,3-b]pyridine,1,3,3-trialkyl-3H-pyrrolo[2,3-b]pyridine such as1,3,3-triethyl-3H-pyrrolo[2,3-b]pyridine, etc.); and athiazolo[4,5-b]quinoline nucleus, a pyrylium (including benzopyrylium,thiapyrylium, and benzothiapyrylium) nucleus, and a dithioliniumnucleus.

In a second preferred embodiment of this invention the sensitizer isrepresented by Formula 2: ##STR3## wherein A, L, Z, k, and m are aspreviously defined, and

R₂ represents hydrogen, nitro, cyano, a carboalkoxy group of from 2 to19 carbon atoms, or halogen.

In a third preferred embodiment of this invention other dyes useful asphotobleachable sensitizers of this invention include5-nitropyrimidyl-4-idene dyes as given in Formula 3.5-Nitropyrimidyl-4-idene dyes contain a 5-nitro-substituted pyrimidylgroup joined at the position through a methine chain linkage to a basicheterocyclic nucleus containing an electron-donating atom, typically anitrogen, oxygen, or sulfur electron-donating atom. Formula 3 is##STR4## wherein R₃ represents halogen; and

A, L, Z, k, and m are defined as above.

In a fourth preferred embodiment, shown by Formula 4, theo-nitro-substituted aryl group is joined through an acyclic methinechain containing an even number of methine groups to a disubstitutedamino group so as to form an enamine. The disubstituted amino group canbe part of a heteroalkyl or heteroaromatic ring which can haveadditional carbocyclic and heterocyclic rings fused thereto. Theo-nitro-substituted aryl group can contain a phenyl or heterocyclicnucleus, or can contain a nucleus formed by fused aromatic orheteroaromatic rings, such as naphthyl and the like. Formula 4 is givenby ##STR5## wherein L and Y are as previously defined; and m is aninteger from 0 to 5, inclusive.

R₄ is chosen from hydrogen, an alkyl group having from 1 to 18 carbonatoms, and preferably a lower alkyl group having from 1 to 4 carbonatoms (e.g., methyl, ethyl, propyl, isopropyl, butyl, sec-butyl,isobutyl, tert-butyl); an aryl group having from 1 to 20 carbon atoms,preferably phenyl; an alkaryl group, preferably lower alkaryl havingfrom 7 to 11 carbon atoms (e.g., tolyl, ethylphenyl, propylphenyl); anaralkyl group, preferably lower aralkyl having from 7 to 11 carbon atoms(e.g., phenylmethyl, phenethyl, phenylpropyl); heteroaromatic (e.g.,pyridyl, pyrimidyl, quinolinyl); an alkoxyalkyl group, preferably alower alkoxy lower alkyl containing from 1 to 4 carbon atoms in both thealkoxy and alkyl moieties (e.g., β-methoxyethyl, γ-methoxypropyl,δ-propoxybutyl, etc.); an alkoxycarbonylalkyl group, preferably a loweralkoxy carbonyl lower alkyl containing from 1 to 4 carbon atoms in boththe alkoxy and alkyl moieties (e.g., β-methoxycarbonylethyl,δ-ethoxycarbonylbutyl, β-butoxycarbonylethyl, etc.); or halogen.

B may be chosen from amino; alkylamino, preferably lower alkylaminohaving from 1 to 6 carbon atoms (e.g., methylamino, ethylamino,propylamino, isopropylamino, hexylamino, etc.); dialkylamino, preferablylower dialkylamino with each alkyl group individually having from 1 to 6carbon atoms (e.g., dimethylamino, diethylamino, ethylmethylamino,propylhexylamino, etc.); cycloalkylamino, preferably lowercycloalkylamino having from 2 to 6 carbon atoms (e.g., cyclopentylamino,cyclohexylamino, etc.); morpholino and substituted morpholinopiperazinoand substituted piperazino; arylamino, preferably phenyl or substitutedphenylamino (e.g., chlorophenylamino, dimethylphenylamino, etc.);diarylamino, preferably with each aryl group individually being phenylor substituted phenyl (e.g., diphenylamino, tolylphenylamino, etc.)

The photobleachable sensitizer should be present as at least 0.05percent by weight of the total weight of the dried imaging layer, up to1.5 percent by weight or more. Preferably, they are present at from0.075 to 1.25 percent by total weight of the layer and most preferablyfrom 0.1 to 1.0 percent.

BINDER

Any natural or synthetic water-insoluble polymeric binder may be used inthe practice of this invention. Organic polymeric resins, preferablythermoplastic resins (although thermoset resins may be used) aregenerally preferred. Where speed is important, water-insoluble, waterimpermeable, water resistant polymers should be used and an acid shouldbe added to the system to increase the rate of colorizing (i.e., leucodye oxidation). Such resins as phenoxy resins, polyesters, polyvinylresins, polycarbonates, polyamides, polyvinyl acetals, polyvinylidenechloride, polyacrylates, cellulose esters, copolymers and blends ofthese classes of resins, and others have been used with particularsuccess. Where the proportions and activities of leuco dyes and nitrateion require a particular developing time and temperature, the resinshould be able to withstand those conditions. Generally, it is preferredthat the polymer not decompose or lose its structural integrity at 200°F. (93° C.), for 30 seconds and most preferred that it not decompose orlose its structural integrity at 260° F. (127° C.).

Preferred polymers include polyvinylidene chloride resins (e.g., Saran™supplied by Dow Chemical Co.), phenoxy resins (e.g., PKHH™ and PAHJ™supplied by Union Carbide Chemical Corp.), and polyvinyl formals (e.g.,Formvar™ supplied by Monsanto Chemical Corp.).

Beyond these minimal requirements, there is no criticality in theselection of a binder. In fact, even transparency and translucency arenot required although they are desirable.

The binder serves a number of additionally important purposes in theconstructions of the present invention. The imageable materials areprotected from ambient conditions such as moisture. The consistency ofthe coating and its image quality are improved. The durability of thefinal image is also significantly improved. The binder should be presentas at least about 25% by weight of ingredients in the layer, morepreferably as 50% or 70% by weight and most preferably as at least about80% by total weight of dry ingredients (i.e., excluding solvents in thelayer). A generally useful range is 30-98% by weight binder with 75-95%preferred.

NITRATE SALT

Nitrate salts themselves are well known. They may be supplied as variouschemical compounds, but are desirably provided as a metal salt, and mostpreferably provided as a hydrated metal salt. Other ions which areordinarily good oxidizing ions such as nitrite, chlorate, iodate,perchlorate, periodate, and persulfate do not provide comparableresults. Extremely active oxidizing agents, such as iodate, even used inrelatively smaller proportions to prevent complete and immediateoxidation or colorization of the dyes do not perform nearly as well asnitrate ion compositions.

The performance of nitrate is so far superior to any other ion that itis apparently unique in the practice of the present invention. Mostmeans of supplying the nitrate salt into the layer are satisfactory, forexample, organic salts, metal salts, acid salts, mixtures of acids andsalts, and other means of supplying the ion are useful. For example,nitrates of zinc, cadmium, potassium, calcium, zirconyl (ZrO₂), nickel,aluminum, chromium, iron, copper, magnesium, lithium, lead and cobalt,ammonium nitrate, cerous ammonium nitrate, and combinations of the abovehave been used.

The nitrate salt component of the present invention is desirably presentin a form within the imaging layer so the oxidizing quantities of HNO₃,NO, NO₂, or N₂ O₄ will be provided within the layer when it is heated toa temperature no greater than 200° C. for 60 seconds and preferably nogreater than 160° C. for 60 or most preferably 30 seconds. This may beaccomplished with many different types of salts, both organic andinorganic, and in variously different types of constructions.

The most convenient way of providing such thermal oxidant providingnitrate salts is to provide a hydrated nitrate salt such as magnesiumnitrate hexahydrate (Mg(NO₃)₂ ×6H₂ O). In addition to hydrated nitratesalts, non-hydrated salts such as ammonium nitrate, pyridinium nitrate,and guanidinium nitrate in an acidic environment are also capable ofproviding the oxidizing capability necessary for practice of the presentinvention.

Besides the inorganic types of salts generally described above, organicsalts in non-alkaline environments are also quite useful in the practiceof the present invention. In particular, nitrated quaternary ammoniumsalts such as guanidinium nitrate work quite well in acid environments,but will not provide any useful image in a basic environment.

It is believed that the alkaline environment causes any oxidizing agent(e.g., HNO₃, NO, NO₂, and/or N₂ O₄) which is liberated from the nitratesalt to be neutralized so as to prevent oxidation of the leuco dyes. Forthis reason it is preferred to have an acidic environment for thenitrate salt.

One other consideration should be given in the selection of the nitratesalt, namely the choice of a salt in which the cation is non-reactivewith the dye. Non-reactive salts are defined in the practice of thepresent invention as those salts whose cations do not spontaneouslyoxidize the dyes that they are associated with at room temperature. Thismay be determined in a number of fashions. For example, the dye and anon-nitrate (preferably halide) salt of the cation may be co-dissolvedin a solution. If the salt oxidizes the dye spontaneously (within twominutes) at room temperature, it is a reactive salt. Such salts assilver nitrate, in which the cation itself is a strong oxidizing agent,is a reactive salt. Ceric nitrate is also reactive, while hydratedcerous nitrate is not.

Preferred salts are the hydrated metal salts such as nickel nitratehexahydrate, magnesium nitrate hexahydrate, aluminum nitratenonahydrate, ferric nitrate nonahydrate, cupric nitrate trihydrate, zincnitrate hexahydrate, cadmium nitrate tetrahydrate, bismuth nitratepentahydrate, thorium nitrate tetrahydrate, cobalt nitrate hexahydrate,gadolinium or lanthanum nitrate nonahydrate, mixtures of these hydratednitrates and the like. Nonhydrated (e.g., lithium nitrate) or organicnitrates may be admixed therewith.

Organic nitrates are also quite useful in the practice of the presentinvention. These nitrates are usually in the form of quarternarynitrogen containing compounds such as guanidinium nitrate, pyridiniumnitrate, and the like. It is preferred to have at least 0.10 moles ofnitrate ion per mole of leuco dye. It is more preferred to have at least0.30 or 0.50 moles of ion per mole of dye. The nitrate ordinarilyconstitutes from 0.05 to 10 percent by weight of the imaging layer,preferably 0.1 to 10 percent and most preferably 0.5 to 8 percent byweight.

LEUCO DYES

Leuco dyes are well known. These are colorless compounds which whensubjected to an oxidation reaction form colored dyes. These leuco dyesare well described in the art (e.g., U.S. Pat. No. 3,974,147; The Theoryof Photographic Process, 3rd Ed.; Mees, C. E. K.; James, R.; MacMillan:New York, 1966; pp 283-284, 390-391; and Kosar, J. Light-SensitiveSystems; John Wiley and Sons: New York, 1965; pp 367, 370-380, 406. Onlythose leuco dyes which can be converted to colored dyes by oxidation areuseful in the practice of the present invention. The preferred leucodyes are the acylated leuco diazine, phenoxazine, and phenothiazine dyesexamples of which are disclosed in U.S. Pat. Nos. 4,460,677, 4,647,525,and G.B. Pat. No. 1,271,289.

Acid or base sensitive dyes such as phenolphthalein and other indicatordyes are not useful in the present invention. Indicator dyes form onlytransient images and are too sensitive to changes in the environment.The leuco dye should be present as at least about 0.3 percent by weightof the total weight of the light-sensitive layer, preferably at least 1percent by weight, and most preferably at least 2 percent to 10 percentor more (e.g. 15 percent) by weight of the dry weight of the imageablelayer. About 10 mole percent of the nitrate/leuco dye is minimally used,with 20 to 80 mole percent preferred and from 35 to 65 mole percent mostpreferred. Molar percentages of nitrate/dye in excess of 100 percent aredefinitely useful. The leuco dye ordinarily constitutes from 0.5 to 15percent by weight of the imaging layer preferably 2 to 8 percent.

INITIATORS

The term initiator as used herein refers to either a diaryliodoniumsalt, or an photolyzable organic halogen compound. Each of the twoclasses of initiators are known in the art.

The diaryliodonium salts of the present invention may be generallydescribed by the formulae: ##STR6## wherein R₅ and R₆ are individuallyselected from aromatic groups. Such aromatic groups may have from 4 to20 carbon atoms (e.g., substituted or unsubstituted phenyl, naphthyl,thienyl, and furanyl) with substantially any substitution,

W is selected from a carbon-carbon bond, oxygen, sulfur, ##STR7##wherein R₇ is aryl (e.g., 6 to 20 carbon atoms) or acyl (e.g., 2 to 20carbon atoms), or R₈ --C--R₉ wherein R₈ and R₉ are selected fromhydrogen, alkyl groups of 1 to 4 carbon atoms, and alkenyl groups of 2to 4 carbon atoms,

p is 0 or 1, and

Q⁻ is any anion.

Where the term group is used in describing substituents, substitution isanticipated on the substituent for example, alkyl group includes ethergroups (e.g., CH₃ --CH₂ --CH₂ --O--CH₂ --), haloalkyls, nitroalkyls,carboxy alkyls, hydroxy alkyls, etc. while the term alkyl includes onlyhydrocarbons. Substituents which react with active ingredients, such asvery strong reducing or oxidizing substituents, would of course beexcluded as not being sensitometrically inert or harmless.

The photolyzable organic halogen compounds are those that upon exposureto radiation dissociate at one or more carbon-halogen bonds to form freeradicals. The carbon-halogen bond dissociation energy should be betweenabout 40 and 70 kilocalories per mole as taught in U.S. Pat. Nos.3,515,552 and 3,536,481. Preferred halogen compounds are nongaseous atroom temperature and have a polarographic half-wave reduction potentialgreater than about -0.9 V as described in U.S. Pat. Nos. 3,640,718,3,617,288, and 3,779,778.

Examples of diaryliodonium cations useful in the practice of the presentinvention are diphenyliodonium, di(4-chlorophenyl)iodonium,4-trifluoromethylphenylphenyliodonium, 4-ethylphenylphenyliodonium,di(4-acetylphenyl)iodonium, tolylphenyliodonium, anisylphenyliodonium,4-butoxyphenylphenyliodonium, di(4-phenylphenyl)iodonium,di(carbomethoxyphenyl)iodonium, etc. Examples of the iodonium cationsare disclosed in U.S. Pat. Nos. 3,729,313, 4,076,705, and 4,386,154.Bis-type forms of these initiators may also be used.

Examples of photolyzable organic halogen compounds are hexabromoethane,α,α,α',α'-tetrabromoxylene, carbon tetrabromide,m-nitro(tribromoacetyl)benzene, α,α,α-trichloroacetanilide,trichloromethylsulfonylbenzene, tribromoquinaldine,bis(pentachlorocyclopentadiene), tribromomethylduinoxaline,α,α-dibromo-p-nitrotoluene, α,α,α,α'α',α'-hexachloro-p-xylene,dibromotetrachloroethane, pentabromoethane, dibromodibenzoylmethane,carbon tetraiodide, halomethyl-s-triazines such as2,4-bis(trichloromethyl)-6-methyl-s-triazine,2,4,6-tris(trichloromethyl)-s-triazine, and2,4-bis(trichloromethyl)-6-(p-methoxystyryl)-s-triazine, etc. Thesecompounds are disclosed as noted above in U.S. Pat. Nos. 3,515,552,3,536,489, 3,617,288, 3,640,718, 4,386,154, and 3,779,778.

ACIDIC MATERIALS

Acidic materials may be added to the light sensitive layer to increaseits speed. The acids used in the present invention are acids asgenerally known to one skilled in the art. Organic acids are preferred,but inorganic acids (generally in relatively smaller concentrations) arealso useful. Organic acids having carboxylic groups are most preferred.The acid should be present as at least about 0.1 percent by weight ofthe total weight of the light sensitive layer. More preferably it ispresent in amounts from 0.2 to 2.0 times the amount of nitrate ion. Theacid may, for example, be present in a range of from 0.05 to 10 percentby weight, preferably from 0.1 to 7 percent, most preferably from 0.5 to5 percent. Higher molecular weight acids are generally used at thehigher concentrations and lower molecular weight acids used at the lowerconcentrations. Anhydrides such as phthalic anhydride may also be used.

In forming or coating imageable layers onto a substrate, temperaturesshould, of course, not be used during manufacture which would completelycolorize the layer or decompose the o-nitroarylidene dyes. Somecolorization is tolerable, with the initial leuco dye concentrationschosen so as to allow for anticipated changes. It is preferred, however,that little or no leuco dye be oxidized during forming and coating sothat more standardized layers can be formed. Depending on theanticipated development temperature the coating or forming temperaturecan be varied. Therefore, if the anticipated development temperaturewere, for example, 220° F. (104° C.), the drying temperature would be140° F. (60° C.). It would therefore not be likely for the layer to gainany of its optical density a drying temperature in less than 6-7minutes. A reasonable development temperature range is between 160° F.(71° C.) and 350° F. (177° C.) and a reasonable dwell time is between 3seconds and 2 minutes, preferably at between 175° F. (79° C.) and 250°F. (121° C.) and for 5 to 60 seconds, with the longer times most likelyassociated with the lower development temperatures.

The imaging layers of the present invention must under some conditionsallow reactive association amongst the active ingredients in order toenable imaging. That is, the individual ingredients may not be separatedby impenetrable barriers (i.e., which cannot be dissolved, broken, ordisrupted during use) within the layer. Generally the active ingredientsare homogeneously mixed (e.g., a molecular mixture) within the layer.They may be individually maintained in heat softenable binders which aredispersed or mixed within the layer and which soften upon heating toallow migration of ingredients, but this would require a longerdevelopment time. The ingredients may be incorporated into a bindermedium, fine particles of which may be subsequently dispersed in asecond layer binder medium. This has similarly been done with dry silverphotothermographic media in U.S. Pat. No. 4,708,928.

The imaging layers of the present invention may contain variousmaterials in combination with the essential ingredients of the presentinvention. For example, plasticizers, coating aids, antioxidants (e.g.,ascorbic acid, hindered phenols phenidone, etc. in amounts that wouldprevent oxidation of the dyes when heated), surfactants, antistaticagents, waxes, ultraviolet radiation absorbers, mild oxidizing agents inaddition to the nitrate, and brighteners may be used without adverselyaffecting the practice of the invention. One noteworthy property of thissystem is amplification of the latent image. By amplification in theleuco dye containing construction is meant that more than one moleculeof dye is formed for each absorbed photon of radiation. The degree ofamplification, that is the ratio of the number of dye molecules formedto photons absorbed, may be as high as 1×10³.

An essential aspect of this invention is that photobleachable spectralsensitizers may be combined with a nitrate salt, leuco dye, andinitiator to provide a light-sensitive thermally developable imagingsystem without residual sensitizer stain in exposed regions afterdevelopment, and that upon a further blanket exposure residualsensitizer stain in unexposed regions can be removed, thereby improvingcontrast, color fidelity over the entire visible spectrum, and lightfastness of the developed imaging system.

Additionally, since the sensitizers of this invention arephotobleachable they may be incorporated at unusually highconcentrations which would be completely unacceptable due to extremelevels of sensitizer stain in the developed image. Photolysis ofphotobleachable sensitizers at these high concentrations can still occurefficiently, since photolyzed material does not serve to filter incidentradiation from unphotolyzed sensitizer. Under identical conditions,non-photobleachable sensitizers are photolyzed to give products whichfilter incident light and prevent absorption by unphotolyzed sensitizer,thereby reducing photoefficiency of the imaging composition.

These and other aspects of the present invention such as the advantagesover the prior art will be shown in the following examples.

EXAMPLES

All materials which were prepared in the following examples wereevaluated and characterized by a selection or combination of ¹ H nuclearmagnetic resonance (NMR), infrared (IR), and ultraviolet (UV)spectroscopies. Spectra obtained were consistent with the assignmentsgiven below. All reagents can be purchased from Aldrich Chemical(Milwaukee, Wis.), unless otherwise specified. The following chemicalabbreviation were used in the examples which follow: ethanol (EtOH),methanol (MeOH), tetrahydrofuran (THF).

The term "appropriate safelights" as used herein means room lightingconditions of such wavelengths that neither the components nor thecomplete imageable layer will be photosensitive to them.

The term "fog" as used herein means the optical density in the unexposedregion.

EXAMPLES 1-6

These examples teach the preparation of some o-nitroarylidene dyesuseful as photobleachable sensitizers in the practice of this invention.

The o-nitroarylidene dyes which are shown in Table 1 were preparedaccording to the general procedures described in U.S. Pat. No.3,988,154.

                  TABLE 1                                                         ______________________________________                                                               λ.sub.max (nm)                                  Example         Dye    in PKHH.sup.a                                          ______________________________________                                        1               1      534                                                    2               2      586                                                    3               3      507                                                    4               4      471                                                    5               5      610                                                    6               6      657                                                    ______________________________________                                         .sup.a PKHH is a phenoxy resin obtained from Union Carbide, Hackensack,       NJ.                                                                      

EXAMPLES 7-18

These examples demonstrate that the photobleachable sensitizing dyes ofExamples 1-6 are useful in the present invention.

A coating solution was made up of the following components: 7.5 g of 20%PKHH™ in THF, 80 mg of the benzoyl leuco of Basic Blue 3 (PergascriptTurquoise™, Ciba-Geigy, Ardsley, N.Y.), 4 to 9 mg sensitizer dye, 940 mgsolution of 9 g MeOH, 0.26 g Mg(NO₃)₂ ×6H₂ O, 0.14 g succinic acid; and60 mg of either 2,4,6-tris(trichloromethyl)-1,3,5-triazine (TTT), ordiphenyliodonium hexafluorophosphate (Ph₂ I).

The solution was placed on a shaker table for 15 minutes at roomtemperature in a dark room. Under appropriate safelights, the solutionwas knife-coated upon 4 mil (0.1 mm) transparent polyethyleneterephthalate film at 4 mil (0.1 mm) wet thickness, and at 66° C. for3.25 minutes. Several duplicate strips (approximately 8"×2") were cutfrom this film and used for the following test.

The development temperature of the dried film was determined by exposinglengthwise one-half of an 8"×2" (20.3×5.1 cm) strip on a 3M brand "179"Contact Printer Processor containing a white tungsten light source for20 seconds at the 32 exposure setting (about 8.5×10⁵ microwatts/cm²).The strip was placed on a Reichert Heizbank thermal gradient apparatus(Cambridge Instruments, Buffalo, N.Y.), for 20 seconds and the thermallimits (the temperature at which development occurs), were determinedfor the exposed (T_(exp)) and unexposed (T_(unexp)) regions. The resultsare presented in Table 2.

                  TABLE 2                                                         ______________________________________                                                                    Thermal Limit                                     Example   Dye    Initiator  T.sub.unexp (°C.)                                                              T.sub.exp (°C.)                    ______________________________________                                         7        1      TTT        83      73                                         8        1      Ph.sub.2 I 94      80                                         9        2      TTT        88      86                                        10        2      Ph.sub.2 I 98      95                                        11        3      TTT        87      72                                        12        3      Ph.sub.2 I 94      80                                        13        4      TTT        88      76                                        14        4      Ph.sub.2 I 100     97                                        15        5      TTT        120     100                                       16        5      Ph.sub.2 I 108     105                                       17        6      TTT        108     100                                       18        6      Ph.sub.2 I 109     107                                       ______________________________________                                    

EXAMPLES 19-30

Duplicate strips to those of Examples 7-18 were subjected to thefollowing tests.

The amount of photobleaching was determined by the following procedure:one half of a strip was exposed on a 3M brand Model 213 OverHeadProjector for 5 minutes. The UV spectrum of the unexposed film wastaken, the λ_(max) was determined, and the optical density of theexposed strip was measured at the λ_(max). The photothermographic speedwas determined by the following procedure: one half of a strip wasexposed lengthwise through a Stouffer √2, 21 step tablet (StoufferGraphic Arts Equipment, South Bend, Ind.), on a 3M brand "179" ContactPrinter Processor or 10 seconds at an exposure setting of 32(approximately 1.4×10⁴ ergs sec/cm² for 450-900 nm). The strip wasprocessed for 20 seconds at a temperature between T_(unexp) and T_(exp)(from Examples 7-18), at which a background density of <0.2 wasobserved. The speed, in number of steps, was determined at the pointwhere the density is 0.6+fog. The results are presented in Table 3.

                  TABLE 3                                                         ______________________________________                                                                  Steps                                               Example Dye     Initiator 0.6 + fog                                                                             % Photobleach                               ______________________________________                                        19      1       TTT       11.0    98                                          20      1       Ph.sub.2 I                                                                              6.5     95                                          21      2       TTT       <2.0    85                                          22      2       Ph.sub.2 I                                                                              <2.0    89                                          23      3       TTT       7.5     96                                          24      3       Ph.sub.2 I                                                                              3.5     94                                          25      4       TTT       6.0     78                                          26      4       Ph.sub.2 I                                                                              3.0     83                                          27      5       TTT       6.0     75                                          28      5       Ph.sub.2 I                                                                              2.0     75                                          29      6       TTT       3.0     88                                          30      6       Ph.sub.2 I                                                                              <2.0    88                                          ______________________________________                                    

EXAMPLE 31

The following example demonstrates that the photobleachable sensitizersof this invention can be different from those of the photothermographicsystems of U.S. Pat. Nos. 4,386,154 and 4,460,677. A test analogous tothe sensitizing dye test specified in U.S. Pat. Nos. 4,386,154 and4,460,677 was performed. A standard test solution was prepared with thefollowing composition: 5.0 g of 5% (weight by volume) solution in methylethyl ketone of polyvinylbutyral (45,000-55,000 molecular weight,9.0-13.0% hydroxyl content, Butvar™-B76, Monsanto Chem. Co., St. Louis,Mo.), 0.3 g of trimethylolpropane trimethacrylate, and 0.03 g of2-methyl-4,6-bis(trichloromethyl)-1,3,5-triazine.

To this solution was added 0.02 g of the indicated dye. The solution wasknife coated onto a 2 mil (0.05 mm) transparent polyethyleneterephthalate film using a knife orifice of 2.0 mil, and the coating wasair dried for 30 minutes. Another 2 mil (0.05 mm) transparentpolyethylene terephthalate film was carefully placed over the dried butsoft and tacky coating with minimum entrapment of air. The sandwichedconstruction was then exposed for 15 seconds to a 3M Model 70 lightsource (650 watt tungsten lamp) through a template with clear and opaqueareas. This procedure essentially photobleached the dyes 1 and 3 in thelight exposed areas. After exposure the cover film was removed, and thecoating was treated with a finely divided black toner powder of the typeconventionally used in xerography. If the tested material was asensitizer as described in U.S. Pat. No. 4,386,154, thetrimethylolpropane trimethacrylate monomer in the light exposed areaswould be polymerized by the light generated free radicals from thephotolyzable organic halogen compound, i.e.,2-methyl-4,6-bis(trichloromethyl)-1,3,5-triazine. Since the polymerizedareas are essentially tack free, the black toner powder selectivelyadheres to the tacky, unexposed areas of the coating, providing a visualimage corresponding to that in the template. The results are shown belowand are compared with a sensitizing dye(5,10-diethoxy-16,17-dimethoxyviolanthrene) of the prior art. Table 4demonstrates the present invention is outside the scope of the prior artas described in U.S. Pat. Nos. 4,386,154 and 4,460,677.

                  TABLE 4                                                         ______________________________________                                        Dye             Toner Image                                                   ______________________________________                                        1               No                                                            3               No                                                            5,10-diethoxy-  Yes                                                           16,17-dimethoxy-                                                              violanthrene                                                                  ______________________________________                                    

EXAMPLES 32-41

These examples teach the preparation of some o-nitropyridyl-2-idene ando,p-dinitropyridyl-2-idene dyes useful as photobleachable sensitizers inthe practice of this invention. A general procedure for the preparationof the o-nitropyridyl-2-idene and the o,p-dinitropyridyl-2-idene dyes7-26 shown in Table 5 follows: to a refluxing solution of 2.5 mmol ofthe appropriate 2- or 4-alkyl quaternary salt (as described in U.S. Pat.No. 4,111,699); 2.5 mol of 2-chloro-3-nitropyridine or2-chloro-3,5-dinitropyridine, and 30 ml dry acetonitrile (distilled fromcalcium hydride) was added 5 ml of diisopropylethylamine over 5 minutesdropwise. The reaction mixture was held at reflux for an additional 5hours. The resulting dark mixture was allowed to cool and standovernight. The precipitate was filtered, washed with acetonitrile, anddried to afford product.

                  TABLE 5                                                         ______________________________________                                        Example        Dye    λ.sub.max (nm)                                   ______________________________________                                        32              7     550 (CH.sub.3 CN)                                       33              8     586 (CH.sub.3 CN)                                       34              9     530 (CH.sub.2 Cl.sub.2)                                 35             10     545 (CH.sub.2 Cl.sub.2)                                 36             11     536 (CH.sub.2 Cl.sub.2)                                 37             12     540 (CH.sub.2 Cl.sub.2)                                 38             13     566 (CH.sub.2 Cl.sub.2)                                 39             14     500 (CH.sub.2 Cl.sub.2)                                 40             15     531 (CH.sub.2 Cl.sub.2)                                 41             16     470 (CH.sub.2 Cl.sub.2)                                 ______________________________________                                    

EXAMPLES 42-59

This example demonstrates that the photobleachable sensitizing dyes ofExamples 32-41 are useful in the present invention. Coated film sampleswere prepared and evaluated according to the procedure of Examples 7-12.The results are presented in Table 6.

                  TABLE 6                                                         ______________________________________                                                                    Thermal Limit                                     Example   Dye    Initiator  T.sub.unexp (°C.)                                                              T.sub.exp (°C.)                    ______________________________________                                        42         7     TTT        100      90                                       43         7     Ph.sub.2 I 125     121                                       44         8     TTT        115     100                                       45         8     Ph.sub.2 I 128     121                                       46         9     TTT        108      95                                       47         9     Ph.sub.2 I 124     120                                       48        10     TTT        120     105                                       49        10     Ph.sub.2 I 118     115                                       50        11     TTT        120      98                                       51        11     Ph.sub.2 I 122     118                                       52        12     TTT        112      98                                       53        12     Ph.sub.2 I 123     121                                       54        13     TTT        120      98                                       55        13     Ph.sub.2 I 122     120                                       56        15     TTT        120      90                                       57        15     Ph.sub.2 I 126     124                                       58        16     TTT        125     110                                       59        16     Ph.sub.2 I 121     118                                       ______________________________________                                    

EXAMPLES 60-69

Strips that were duplicates to those of Examples 42-59 were subjected tophotobleaching and speed tests as described in Examples 19-30. Theresults are presented in Table 7.

                  TABLE 7                                                         ______________________________________                                                                  Steps                                               Example Dye     Initiator 0.6 + fog                                                                             % Photobleach                               ______________________________________                                        60       7      TTT       4       99                                          61       8      TTT       3       97                                          62       9      TTT       4       94                                          63      10      TTT       3       23                                          64      11      TTT       5       21                                          65      12      TTT       5       53                                          66      13      TTT       2       21                                          67      14      TTT       6       68                                          68      15      TTT       4       20                                          69      16      TTT       2       20                                          ______________________________________                                    

EXAMPLE 70-77

These examples teach the preparation of some o-nitropyrimidyl-4-idenedyes useful as photobleachable sensitizers in the practice of thisinvention.

General procedure for the o-nitropyrimidyl-4-idene dyes 17-24: to arefluxing solution of 5.2 mmol of the appropriate 2- or 4-alkylquaternary salt (as described in U.S. Pat. No. 4,111,699); 5.2 mol of4,6-dichloro-5-nitropyrimidine, and 30 ml dry acetonitrile (distilledfrom calcium hydride) was added 5 ml diisopropylethylamine dropwise over5 minutes. The reaction mixture was held at reflux for an additional 5hours. The resulting dark mixture was allowed to cool and standovernight. The precipitate is filtered, washed with acetonitrile, anddried to afford product listed in Table 8.

                  TABLE 8                                                         ______________________________________                                        Example        Dye    λ.sub.max (nm)                                   ______________________________________                                        70             17     478 (CH.sub.2 Cl.sub.2)                                 71             18     499 (CH.sub.2 Cl.sub.2)                                 72             19     450 (CH.sub.2 Cl.sub.2)                                 73             20     436 (CH.sub.2 Cl.sub.2)                                 74             21     449 (CH.sub.2 Cl.sub.2)                                 75             22     404 (CH.sub.2 Cl.sub.2)                                 ______________________________________                                    

EXAMPLES 78-93

These examples demonstrate that the photobleachable sensitizing dyes ofExamples 70-75 are useful in the present invention. Coated film sampleswere prepared and evaluated according to the procedure of Examples 7-12.The results are presented in Table 9.

                  TABLE 9                                                         ______________________________________                                                                     Thermal                                                                              Limits                                    Example  Sensitizer                                                                             Initiator  T.sub.H (°C.)                                                                 T.sub.L (°C.)                      ______________________________________                                        78       17       TTT        112    106                                       79       17       Ph.sub.2 I 123    120                                       80       18       TTT        120    112                                       81       18       Ph.sub.2 I 122    120                                       82       19       TTT        109     97                                       83       19       Ph.sub.2 I 125    121                                       84       20       TTT        110     95                                       85       20       Ph.sub.2 I 120    118                                       86       21       TTT        125    108                                       87       21       Ph.sub.2 I 122    120                                       88       22       TTT        112     98                                       89       22       Ph.sub.2 I 121    119                                       ______________________________________                                    

EXAMPLES 90-95

Strips that were duplicates to those of Examples 78-89 were subjected tophotobleaching and speed tests as described in Examples 19-30. Theresults are presented in Table 10.

                  TABLE 10                                                        ______________________________________                                                                  Steps                                               Example Dye     Initiator 0.6 + fog                                                                             % Photobleach                               ______________________________________                                        90      17      TTT       4       84                                          91      18      TTT       3       94                                          92      19      TTT       8       92                                          93      20      TTT       6       24                                          94      21      TTT       2       30                                          95      22      TTT       5       69                                          ______________________________________                                    

EXAMPLES 96 AND 97

These examples teach the preparation of some o,p-dinitrophenyl enamineswhich are useful as photobleachable sensitizers in the practice of thepresent invention.

Compound 23 is prepared from 2,4-dinitrofluorobenzene and the morpholineenamine of acetophenone (conditions to be supplied). Compound 24 iscommercially available from Aldrich Chemical.

                  TABLE 11                                                        ______________________________________                                                               λ.sub.max (nm)                                  Example         Dye    in PKHH                                                ______________________________________                                        96              23     440-50                                                 97              24     469                                                    ______________________________________                                    

EXAMPLE 98

This example demonstrates that the photobleachable sensitizing dyes ofExamples 96 and 97 are useful in the present invention. Films ofcompounds 23 and 24 were prepared by dissolving dye 23 (0.034 g) or dye24 (0.02 g) in 7.5 of 20% PKHH binder in THF and coating at 4 mil (0.1mm) wet thickness onto 4 mil (0.1 mm) transparent polyethyleneterephthalate film followed by oven drying at 50° C. for 5 minutes.

When employed as sensitizers in the compositions of Example 8, dye 23with triazine initiator (TTT), afforded a negative image with a 4° to 6°C. differential with 91% photobleaching according to the method ofExamples 19-30, and dye 24 afforded a 5° to 7° C. differential with 84%photobleaching.

EXAMPLES 99-102

These examples demonstrate that o-nitroarylidene dye 1 activates thethermal development in exposed areas. Solution A was prepared by mixing26.25 g of 20% PKHH in THF, 0.28 g of the benzoyl leuco of Basic Blue 3,and 0.04 g of dye 1.

Solution B was prepared as solution A but without any added 1. SolutionC was prepared by mixing 9 g of methanol, 0.26 g Mg(NO₃)₂ ×6H₂ O, and0.14 g of succinic acid.

Four coating solutions were prepared by mixing 7.5 g of either A or B,0.94 g of solution C, and 0.06 of either TTT or Ph₂ I. Coatings wereprepared according to the procedure of Examples 7-18. Strips (20.2×5.1cm) were placed on the Reichert Heizbank apparatus for 20 seconds, andthe thermal limit was determined. The results are presented in Table 12.

                  TABLE 12                                                        ______________________________________                                                                       Thermal                                        Example  Initiator   Dye 1     Limit (°C.)                             ______________________________________                                         99      TTT         not present                                                                              94                                            100      TTT         present    93                                            101      Ph.sub.2 I  not present                                                                             105                                            102      Ph.sub.2 I  present   104                                            ______________________________________                                    

Strips of the films from Examples 100 and 102 were imagewise exposed for10 seconds on a 3M brand Model "179" Contact Printer Processor at the 32exposure setting and afforded average thermal readings on the Heizbankof 85° C. and 97° C., respectively.

EXAMPLES 103-104

These examples demonstrate that the light sensitive layers of thepresent invention may be photobleached, following thermal development,to produce color images with lower background stain and improved colorpurity.

Films were prepared according to Examples 100 and 102. The imagewiseexposed films were then thermally processed for 20 seconds at theindicated temperature to afford bright blue-cyan colored negative imagesof the original with magenta stain throughout. Transmission densitometerreadings (Status A green indicative of magenta color, and Status A redindicative of cyan color), were determined for both D_(max) (lightexposed), and D_(min) (unexposed) areas. The imaged and thermallyprocessed samples were exposed for 1 minute on a 3M brand Model 213Overhead Projector to afford a bright cyan image with much lowerbackground stain. D_(min) and D_(max) densitometry readings weremeasured again.

The results are presented in Table 13 and show the improvement in Dminand color purity by post-exposure bleaching.

                  TABLE 13                                                        ______________________________________                                                           Development                                                Example   Initiator                                                                              Temp. (°C.)                                                                         D.sub.min                                                                          D.sub.max                                ______________________________________                                        103       TTT      84           0.11 1.6.sup.a                                                                0.54 0.68.sup.b                                                               0.07 0.19.sup.c                               104       Ph.sub.2 I                                                                             92           0.10 1.1.sup.a                                                                0.32 0.44.sup.b                                                               0.08 0.19.sup.c                               ______________________________________                                         .sup.a Initial Status A red filter, prior to postexposure step.               .sup.b Initial Status A green filter, prior to postexposure step.             .sup.c Initial Status A green filter, following postexposure step.       

EXAMPLES 105-108

These examples demonstrate image stabilization (i.e., fixing) of theimaged and processed samples using a basic material.

The imaged, processed, and post-exposure photobleached samples ofExamples 103 and 104 were used. Strips of D_(min) regions of the samples(areas which has not been initially photoexposed with the imaging lightsource, and thus were essentially colorless), were used. For each film,one strip was placed for 5 minutes in the vapors inside a tankcontaining 30% aqueous ammonium hydroxide solution, and the other stripwas not. The strips were then placed on a Reichert Heizbank apparatusfor 20 seconds and the thermal limits were measured. Also, Status A reddensitometer readings (indicative of image-dye fog), were determined inareas that had been in contact with 75° C. and 80° C. regions of theHeizbank. The results are presented in Table 14.

                  TABLE 14                                                        ______________________________________                                                                   Thermal                                                                       Limit                                              Example Initiator                                                                              NH.sub.3 fix                                                                            (°C.)                                                                          Fog.sup.a                                                                           Fog.sup.b                            ______________________________________                                        105     TTT      none      80      0.44  1.31                                 106     TTT      5 min.    >150    0.12  0.12                                 107     Ph.sub.2 I                                                                             none      88      0.26  0.36                                 108     Ph.sub.2 I                                                                             5 min.    145     0.09  0.10                                 ______________________________________                                         .sup.a 75° C. region.                                                  .sup.b 80° C. region.                                                  ##STR8##

What is claimed is:
 1. A negative-acting, photothermographic imageablelayer comprising a binder, at least one leuco dye, nitrate ion, anultraviolet radiation sensitive initiator, and a photobleachablespectral sensitizer for said initiator, said sensitizer bleaching atleast 50% under the following conditions: a film of polyethyleneterephthalate (4 mil thickness) is coated with the sensitizer inquestion so as to create a colored film with an absorbance of from 0.1to 0.6, whereupon said colored film is then exposed to light from aprojection bulb having a color temperature of 33300° K., and the lightintensity on the colored film is 0.46 W/cm² ±0.05 W/cm².
 2. Theimageable layer of claim 1 in which the ultraviolet radiation sensitiveinitiator is a diaryliodonium salt.
 3. The imageable layer of claim 1 inwhich the ultraviolet light sensitive initiator is an organic compoundhaving at least one photolyzable halogen atom.
 4. The layer of claim 3wherein said sensitizer is a photobleachable arylidene dye.
 5. Theimageable layer of claim 1 in which an organic acid is present inaddition to the binder, leuco dye, nitrate ion, initiator, andphotobleachable spectral sensitizer.
 6. A process for forming an imagecomprising exposing the imageable layer of claim 1 to visible radiationwithin a range absorbed by said photobleachable spectral sensitizer, andheating the exposed layer to develop an image.
 7. A process according toclaim 6 in which the background photobleachable spectral sensitizerstain of the developed image is reduced comprising exposing the imagedeveloped after heating and exposer to a blanket irradiation of visiblelight.
 8. A process according to claim 7 in which a developed image isfixed comprising exposing said developed image to a source of basicmaterial after exposure to a blanket irradiation of visible light.
 9. Anegative-acting photothermographic element comprising a substrate havingcoated on at least one surface thereof a layer of claim
 1. 10. Theelement of claim 9 wherein said initiator comprises a photosensitivediaryliodonium salt.
 11. The element of claim 9 wherein said initiatorcomprises an organic compound having at least one photolyzable halogenatom.
 12. A negative-acting, photothermographic imageable layercomprising a binder, at least one leuco dye, nitrate ion, an ultravioletradiation sensitive initiator elected from the group consisting of anorganic compound having at least one photolabile hydrogen and adiaryliodonium salt, and a photobleachable spectral sensitizer for saidinitiator, said sensitizer bleaching at least 50% under the followingconditions:a film of polyethylene terephthalate (4 mil thickness) iscoated with the sensitizer in question so as to create a colored filmwith an absorbance of from 0.1 to 0.6, whereupon said colored film isthen exposed to light from a projection bulb having a color temperatureof 33300° K., and the light intensity on the colored film is 0.46 W/cm²±0.05 W/cm², wherein the photobleachable spectral sensitizer isrepresented by the formula: ##STR9## wherein k represents 0 or 1;mrepresents 0 or 1; each L represents a methine group, includingsubstituted groups; A represents an electron donating moiety, such asoxygen (--O--), sulfur (--S--), or ##STR10## R₁ is selected from thegroup consisting of an alkyl group having from 1 to 18 carbon atoms; asulfoalkyl group having from 1 to 4 carbon sulfatoalkyl group having 1to 4 carbon atoms; an alkoxyalkyl group having from 2 to 5 carbon atoms;an acyloxyalkyl group having from 2 to 5 carbon atoms; analkoxycarbonylalkyl group having 1 to 4 carbon atoms in both the alkoxyand alkyl moieties; a dialkylaminoalkylene group having 2 to 8 carbonatoms; a cycloaminoalkylene group having 4 to 6 atoms in the cycloaminomoiety an 1 to 4 atoms in the alkyl moiety; an alkyneyl group having 2to 4 carbon atoms; a substituted or unsubstituted aryl group having from1 to 20 carbon atoms, further wherein the substituent is chosen from thegroup consisting of halogen, alkoxy having from 1 to 4 carbon atoms,alkyl having from 1 to 4 carbon atoms; or hydrogen; and Y represents theatoms necessary to complete an aryl ring which is o-nitro-substitutedand preferably is also p-substituted with a nitro or other electronwithdrawing group and which aryl ring can have other substituentsattached to it and other carbocyclic rings fused to it; and Z representsthe nonmetallic atoms necessary to complete a heterocyclic nucleus ofthe type used in cyanine dyes containing 5 or 6 atoms in theheterocyclic ring containing the electron-donating atom of the formulawhich ring can contain a second heteroatom such as oxygen, nitrogen,selenium, or sulfur, the heterocyclic nucleus being selected from thegroup consisting of thiazole nucleus including substituted andunsubstituted benzothiazole and naphthothiazole nuclei; an oxazolenucleus including substituted and unsubstituted benzoxazole andnaphthoxazole nuclei; a selenazole nucleus including substituted orunsubstituted benzoselenazole and naphthoselenazole nuclei; a thiazolinenucleus; a 4-pyridine nucleus; a 3,3-dialkylindolenine nucleus; animidazole nucleus; a quinoline nucleus; an imidazo[4,5-b]quinolinenucleus; a pyrylium nucleus; and a dithiolinium nucleus.
 13. Anegative-acting, photothermographic imageable layer comprising a binder,at least one leuco dye, nitrate ion, an ultraviolet radiation sensitiveinitiator elected from the group consisting of an organic compoundhaving at least one photolabile hydrogen and a diaryliodonium salt, anda photobleachable spectral sensitizer for said initiator, saidsensitizer bleaching at least 50% under the following conditions:a filmof polyethylene terephthalate (4 mil thickness) is coated with thesensitizer in question so as to create a colored film with an absorbanceof from 0.1 to 0.6, whereupon said colored film is then exposed to lightfrom a projection bulb having a color temperature of 33300° K., and thelight intensity on the colored film is 0.46W/cm² ±0.05W/cm², wherein thephotobleachable spectral sensitizer is represented by the formula##STR11## wherein each L represents a methine group, includingsubstituted groups;A represents an electron donating moiety, such asoxygen (--O--), sulfur (--S--), or ##STR12## R₁ is selected from thegroup consisting of an alkyl group having from 1 to 18 carbon atoms; asulfoalkyl group having from 1 to 4 carbon sulfatoalkyl group having 1to 4 carbon atoms; an alkoxyalkyl group having from 2 to 5 carbon atoms;an acyloxyalkyl group having from 2 to 5 carbon atoms; analkoxycarbonylakyl group having 1 to 4 carbon atoms in both the alkoxyand alkyl moieties; a dialkylaminoalkylene group having 2 to 8 carbonatoms; a cycloaminoalkylene group having 4 to 6 atoms in the cycloaminomoiety an 1 to 4 atoms in the alkyl moiety; an alkyneyl group having 2to 4 carbon atoms; a substituted or unsubstitued aryl group having from1 to 20 carbon atoms, further wherein the substituent is chosen from thegroup consisting of halogen, alkoxy having from 1 to 4 carbon atoms,alkyl having from 1 to 4 carbon atoms; or hydrogen; and Y represents theatoms necessary to complete an aryl ring which is o-nitro-substitutedand preferably is also p-substituted with a nitro or other electronwithdrawing group and which aryl ring can have other substituentsattached to it and other carbocyclic rings fused to it; and Z representsthe nonmetallic atoms necessary to complete a heterocyclic nucleus ofthe type used in cyanine dyes containing 5 or 6 atoms in the heterocylicring containing the electron-donating atom of the formula which ring cancontain a second heteroatom selected from the group consisting ofoxygen, nitrogen, selenium, or sulfur; and R₂ represents hydrogen,nitro, cyano, a carboalkoxy group of from 2 to 19 carbon atoms, orhalogen.
 14. A negative-acting, photothermographic imageable layercomprising a binder, at least one leuco dye, nitrate ion, an ultravioletradiation sensitive initiator elected from the group consisting of anorganic compound having at least one photolabile hydrogen and adiaryliodonium salt, and a photobleachable spectral sensitizer for saidinitiator, said sensitizer bleaching at least 50% under the followingconditions:a film of polyethylene terephthalate (4 mil thickness) iscoated with the sensitizer in question so as to create a colored filmwith an absorbance of from 0.1 to 0.6, whereupon said colored film isthen exposed to light from a projection bulb having a color temperatureof 33300° K., and the light intensity on the colored film is 0.46W/cm²±0.05W/cm², wherein the photobleachable spectral sensitizer isrepresented by the formula ##STR13## wherein R₃ represents halogen; andeach L represents a methine group, including substituted groups;arepresents an electron donating moiety, such as oxygen (--O--), sulfur(--S--), or ##STR14## R₁ is selected from the group consisting of analkyl group having from 1 to 18 carbon atoms; a sulfoalkyl group having1 to 4 carbon atoms; an alkoxyalkyl group having 1 to 4 carbon atoms; analkoxyalkyl group having from 2 to 5 carbon atoms; an acyloxyalkyl grouphaving from 2 to 5 carbon atoms; an alkoxycarbonylalkyl group having 1to 4 carbon atoms in both the alkoxy and alkyl moieties; adialkylaminoalkylene group having 2 to 8 carbon atoms; acycloaminoalkylene group having 4 to 6 atoms in the cycloamino moiety an1 to 4 atoms in the alkyl moiety; an alkyneyl group having 2 to 4 carbonatoms; a substituted or unsubstitued aryl group having from 1 to 20carbon atoms, further wherein the substituent is chosen from the groupconsisting of halogen, alkoxy having from 1 to 4 carbon atoms, alkylhaving from 1 to 4 carbon atoms; or hydrogen; and Y represents the atomsnecessary to complete an aryl ring which is o-nitro-substituted and mayis also p-substituted with a nitro or other electron withdrawing groupand which aryl ring can have other substituents attached to it and othercarbocyclic rings fused to it; and Z represents the nonmetallic atomsnecessary to complete a heterocyclic nucleus of the type used in cyaninedyes containing 5 or 6 atoms in the heterocylic ring containing theelectron-donating atom of the formula which ring can contain a secondheteroatom such as oxygen, nitrogen, selenium, or sulfur
 15. Anegative-acting, photothermographic imageable layer comprising a binder,at least one leuco dye, nitrate ion, an ultraviolet radiation sensitiveinitiator elected from the group consisting of an organic compoundhaving at least one photolabile hydrogen and a diaryliodonium salt, anda photobleachable spectral sensitizer for said initiator, saidsensitizer bleaching at least 50% under the following conditions:a filmof polyethylene terephthalate (4 mil thickness) is coated with thesensitizer in question so as to create a colored film with an absorbanceof from 0.1 to 0.6, whereupon said colored film is then exposed to lightfrom a projection bulb having a color temperature of 33300° K., and thelight intensity on the colored film is 0.46W/cm² ±0.05W/cm², wherein thephotobleachable spectral sensitizer is represented by the formula##STR15## wherein each L represents a methine group, includingsubstituted groups;Y represents the atoms necessary to complete an arylring which is o-nitro-substituted and preferably is also p-substitutedwith a nitro or other electron withdrawing group and which aryl ring canhave other substituents attached to it and other carbocyclic rings fusedto it; and m is an integer from 0 to 5, inclusive; and R₄ is selectedfrom the group consisting of hydrogen, alkyl having from 1 to 18 carbonatoms; aryl having from 1 to 20 carbon atoms; alkaryl having from 7 to11 carbon atoms; aralkyl having from 7 to 11 carbon atoms; heteroaryl;alkoxyalkyl containing from 1 to 4 carbon atoms in each of the alkoxyand alkyl moieties; alkoxycarbonylalkyl containing from 1 to 4 carbonatoms in each of the alkoxy and alkyl moieties; or halogen; and B may bechosen from amino; alkylamino having from 1 to 6 carbon atoms;dialkylamino with each alkyl group individually having from 1 to 6carbon atoms; cycloalkyamino having from 2 to 6 carbon atoms;morpholino; alkylmorpholino having from 4 to 18 carbon atoms;alkylpiperazino having from 5 to 18 carbon atoms; arylamino having from6 to 10 carbon atoms; diarylamino with each aryl group individuallybeing phenyl or substituted phenyl, and having from 6 to 18 carbonatoms.
 16. A negative-acting photothermographic element comprising asubstrate having coated on at least one surface thereof in reactiveassociation in one or more layers of binder: 1) a leuco dye, 2) nitrateion, 3) ultraviolet radiation sensitive initiator, and 4) a visiblelight spectral sensitizer for said initiator, wherein said sensitizer isa photobleachable arylidene dye.
 17. A negative-acting,photothermographic imageable layer comprising a binder, at least oneleuco dye, nitrate ion, an ultraviolet radiation sensitive initiator,and a photobleachable spectral sensitizer for said initiator, saidsensitizer bleaching at least 25% under the following conditions:a filmof polyethylene terephthalate (4 mil thickness) is coated with thesensitizer in question so as to create a colored film with an absorbanceof from 0.1 to 0.6, whereupon said colored film is then exposed to lightfrom a projection bulb having a color temperature of 33300° K., and thelight intensity on the colored film is 0.46W/cm² ±0.05W/cm².
 18. Theimageable layer of claim 17 in which the ultraviolet radiation sensitiveinitiator is a diaryliodonium salt.
 19. A negative-acting,photothermographic imageable layer comprising a binder, at least oneleuco dye, nitrate ion, an ultraviolet radiation sensitive initiatorelected from the group consisting of an organic compound having at leastone photolabile hydrogen and a diaryliodonium salt, and a photoblechablespectral sensitizer for said initiator, said sensitizer bleaching atleast 50% under the following conditions:a film of polyethyleneterephthalate (4 mil thickness) is coated with the sensitizer inquestion so as to create a colored film with an absorbance of from 0.1to 0.6, whereupon said colored film is then exposed to light from aprojection bulb having a color temperature of 33300° K., and the lightintensity on the colored film is 0.46W/cm² ±0.05W/cm², wherein saidsensitizer is a photobleachable arylidene dye.
 20. A negative-acting,photothermographic imageable layer comprising a binder, at least oneleuco dye, nitrate ion, an ultraviolet radiation sensitive initiatorelected from the group consisting of an organic compound having at leastone photolabile hydrogen and a diaryliodonium salt, and aphotobleachable spectral sensitizer for said initiator, said sensitizerbleaching at least 50% under the following conditions:a film ofpolyethylene terephthalate (4 mil thickness) is coated with thesensitizer in question so as to create a colored film with an absorbanceof from 0.1 to 0.6, whereupon said colored film is then exposed to lightfrom a projection bulb having a color temperature of 33300° K., and thelight intensity on the colored film is 0.46W/cm² ±0.05W/cm², in whichthe ultraviolet radiation sensitive initiator is a diaryliodonium salt,and in which an organic acid is present in addition to the binder, leucodye, nitrate ion, initiator, and sensitizer, and the photobleachablespectral sensitizer is represented by the formula ##STR16## wherein krepresents 0 or 1;m represents 0 or 1; each L represents a methinegroup; A represents an electron donating moiety, R₁ is selected from thegroup consisting of an alkyl group having from 1 to 18 carbon atoms; asulfoalkyl group having from 1 to 4 carbon sulfatoalkyl group having 1to 4 carbon atoms; an alkoxyalkyl group having from 2 to 5 carbon atoms;an acyloxyalkyl group having from 2 to 5 carbon atoms; analkoxycarbonylalkyl group having 1 to 4 carbon atoms in both the alkoxyand alkyl moieties; a dialkylaminoalkylene group having 2 to 8 carbonatoms; a cycloaminoalkylene group having 4 to 6 atoms in the cycloaminomoiety an 1 to 4 atoms in the alkyl moiety; an alkyneyl group having 2to 4 carbon atoms; a substituted or unsubstituted aryl group having from1 to 20 carbon atoms, further wherein the substituent is chosen from thegroup consisting of halogen, alkoxy having from 1 to 4 carbon atoms,alkyl having from 1 to 4 carbon atoms; or hydrogen; and Y represents theatoms necessary to complete an aryl ring which is o-nitro-substitutedand preferably is also p-substituted with a nitro or other electronwithdrawing group and which aryl ring can have other substituentsattached to it and other carbocyclic rings fused to it; and Z representsthe nonmetallic atoms necessary to complete a heterocyclic nucleus ofthe type used in cyanine dyes containing 5 or 6 atoms in the heterocylicring containing the electron-donating atom of the formula which ring cancontain a second heteroatom such as oxygen, nitrogen, selenium, orsulfur, the heterocyclic nucleus being selected from the groupconsisting of thiazole nucleus including substituted and unsubstitutedbenzothiazole and naphthothiazole nuclei; an oxazole nucleus includingsubstituted and unsubstituted benzoxazole and naphthoxazole nuclei; aselenazole nucleus including substituted or unsubstitutedbenzoselenazole and naphthoselenazole nuclei; a thiazoline nucleus; a4-pyridine nucleus; a 3,3-dialkylindolenine nucleus; an imidazolenucleus; a quinoline nucleus; an imidazo quinoline nucleus; a pyryliumnucleus; and a dithiolinium nucleus.